1. Field of the Invention
The present invention relates to a digital camera having an image pick-up device such as a CCD image sensor, and more specifically relates to such a digital camera which includes an improved structure for fixing the image pick-up device that is to be included in a camera body of the digital camera.
2. Description of the Related Art
A CCD image sensor is widely used as an image pick-up device (image sensor) of a digital camera. This type of CCD image sensor is usually provided in the form of a CCD package (image sensor package), and this CCD package is included in a camera body, mounted thereto. The CCD package is constructed so that a CCD chip (CCD image sensor) is fixed to a package base by adhesive or solder and so that the CCD chip is electrically connected to external leads provided on the package base. A protection glass plate is fixed to the front of the package base to seal the CCD chip between the protection glass plate and the package base. The CCD package which is structured in such a manner is installed at a position (image forming position) inside the camera body in the vicinity of the rear surface thereof where an image is formed through a photographing optical system. It is generally the case that the CCD package, together with a low-pass filter and other components which are to be positioned in front of the CCD image sensor, is mounted to a reference plate to constitute a CCD unit (image sensor unit) and that this CCD unit is fixed to the camera body via the reference plate. Additionally, a light receiving surface (imaging surface) of the CCD chip is oriented to be orthogonal to the optical axis of the photographing optical system at a position of an image plane, on which an image is formed through a photographing optical system, usually at a focal point of the photographing optical system.
Such a conventional type of CCD unit is made by bonding a rear surface of the package base of the CCD package to a front surface (fixing surface) of the reference plate by adhesive when the CCD package is mounted to the reference plate. Fixing the reference plate of this CCD unit to a camera body at a given position thereon makes it possible for the reference plate to be installed with the reference plate being orthogonal to the optical axis of the photographing optical system, and makes it possible for the imaging surface of the CCD chip to be positioned orthogonal to the optical axis of the photographing optical system at a focal point thereon, and further makes it possible to determine the focal point.
However, it is often the case that adhesive or solder by which the CCD chip is fixed to the package base is not evenly coated on the package base. Accordingly, a CCD package in which the CCD chip is bonded to the package base with the imaging surface of the CCD chip not being precisely parallel to the package base is often produced. Therefore, even if the CCD package is bonded to the reference plate of the CCD unit, the CCD unit in which the imaging surface of the CCD chip is precisely parallel to the reference plate cannot be obtained. Consequently, the imaging surface of the CCD chip cannot be oriented in a position precisely orthogonal to the optical axis of the photographing optical system, so that the imaging surface of the CCD chip is slightly inclined to the focal plane. Moreover, the focal point does not coincide with the imaging surface of the CCD chip.
In a digital camera using a small-format CCD image sensor as a CCD chip incorporated in the CCD package, in which the diagonal size of the imaging surface of the CCD chip is equal to or smaller than ½ inch, the aperture of an imaging lens is small so as to correspond to the diagonal size of the imaging surface of the CCD chip while the imaging angle on the imaging surface with respect to the optical axis of the imaging lens is small. In contrast to such a digital camera, in a digital camera using a large-format CCD image sensor as a CCD chip incorporated in the CCD package, in which the diagonal size of the imaging surface of the CCD chip is equal to or greater than 4/3 inches, the aperture of an imaging lens is large and the imaging angle on the imaging surface with respect to the optical axis of the imaging lens is large. Accordingly, the depth of focus in the case of using the large-format CCD image sensor is narrower than the depth of focus in the case of using the small-format CCD image sensor. Therefore, if the CCD package is fixed to a camera body with the imaging surface of the CCD chip being slightly inclined to a plane orthogonal to the optical axis of a photographing optical system, an image that is formed on a periphery of the imaging surface, a deviation of which from a focal plane in the optical axis direction is greater than a deviation of a central portion of the imaging surface from the focal plane in the optical axis direction, easily becomes out-of-focus (blurred); the amount of defocus becomes conspicuous specifically in the case of using the large-format CCD image sensor, in which the deviation in the optical axis direction in the periphery of the imaging surface becomes great. Accordingly, in the case of using the large-format CCD image sensor, a positional deviation of the imaging surface cannot be tolerated in the depth of field of a photographing lens.
As can be understood from the above description, a conventional CCD unit is not always constructed so that the imaging surface of the CCD chip is precisely parallel to the reference plate. Therefore, a structure for positioning the imaging surface of the CCD chip so that the imaging surface becomes orthogonal to the photographing optical axis is required when the CCD unit is fixed to a camera body. For instance, a structure fixing the CCD unit to a camera body has been proposed in Japanese Unexamined Patent Publication 2003-69886. According to this fixing structure, a camera body is provided with a plurality of screw holes, a corresponding plurality of adjusting cylindrical members are respectively screwed into the plurality of screw holes, and a reference plate of the CCD unit is fixed to the plurality of adjusting cylindrical members by set screws with the reference plate contacting with end surfaces of the plurality of adjusting cylindrical members. This fixing structure makes it possible to adjust the inclination of the reference plate so that the imaging surface of the CCD chip becomes precisely orthogonal to the photographing optical axis by varying the height (axial position) of the end surface of each adjusting cylindrical member by changing the rotational position of each adjusting cylindrical member while viewing the image captured by the imaging surface of the CCD chip.
Providing a camera body with such a conventional fixing structure increases the number of elements of a camera, and also requires that an adjusting operation be performed in which the aforementioned plurality of adjusting cylindrical members are adjusted while it is visually checked whether the imaging surface of the CCD chip is directed toward a predetermined direction when the CCD unit is installed in the camera body. Accordingly, the aforementioned conventional fixing structure complicates the operation installing the CCD unit in a camera body, which is in need of improvement.